1. Field of the Invention
The present invention relates to driving techniques.
2. Description of the Background Art
Various types of drive devices using electromechanical transducers such as piezoelectric elements have been proposed.
For example, in a fixed element type drive device schematically shown in FIGS. 21A to 21C, a piezoelectric element 92 which is an electromechanical transducer is fixed at its one end in directions of expansion and contraction to a fixed member 91 and at the other end to a driven friction member 94. The driven friction member 94 moves in forward and backward directions with expansion and contraction of the piezoelectric element 92. The driven friction member 94 is engaged with a moving part 93 by a frictional force.
The moving part 93 is driven by application of voltage to the piezoelectric element 92 to cause the piezoelectric element 92 to expand and contract at different speeds. FIGS. 21A to 21C show the conditions at times Pa, Pb, and Pc, respectively, upon application of voltage having a forward waveform shown in FIG. 22.
For a gradual rise of the voltage waveform during a time interval between Pa and Pb in FIG. 22, the piezoelectric element 92 expands relatively slowly so that the condition of FIG. 21A transitions to the condition of FIG. 21B. At this time, the moving part 93 substantially moves integrally with the driven friction member 94 without or almost without sliding over the driven friction member 94.
Subsequently, for a rapid fall of the voltage waveform during a time interval between Pb and Pc, the piezoelectric element 92 contracts relatively rapidly so that the driven friction member 94 rapidly returns to its initial position. At this time, slippage occurs between the driven friction member 94 and the moving part 93, so that only the driven friction member 94 returns to its initial position with no substantial movement of the moving part 93. The result is that the moving part 93, as shown in FIG. 21C, moves in the forward direction from its initial position of FIG. 21A.
By repetitions of this cycle, the moving part 93 is moved along the driven friction member 94.
It is noted that the moving part 93 moves in the backward direction upon application of voltage having a backward waveform with steep rising edges and gentle falling edges to the piezoelectric element 92.
There are the following two techniques for applying a sawtooth voltage to the piezoelectric element 92.
FIGS. 23A to 23C illustrate the first technique. Referring to FIG. 23A, for example an 8-bit sawtooth voltage of 0 to 5 V is generated by a digital analog converter of a waveform generator 95 and is input to a power amplifier 96, in which then the sawtooth voltage is amplified to, for example, 0 to 10 V for drive and applied to a piezoelectric element Pv. By controlling the waveform generator 95, both a forward sawtooth waveform shown in FIG. 23B and a backward sawtooth waveform shown in FIG. 23C can be generated.
FIG. 24 and FIGS. 25A and 25B illustrate the second technique. Referring to FIG. 24, for application of voltage of a power supply 97 to a piezoelectric element Pv, a circuit including constant-current circuits 98a and 98b and switching circuits 99a and 99b is used, in which the constant-current circuits 98a and 98b and the switching circuits 99a and 99b are alternately operated to generate forward and backward waveforms.
More specifically, for example a digital circuit shown in FIG. 25A is configured, in which control signals as shown in FIG. 25B are input to terminals Ra to Rd to thereby generate forward and backward waveforms.
In other words, after a HIGH signal is input to the terminal Ra to gradually raise voltage applied through the constant-current circuit 98a to the piezoelectric element Pv, then a HIGH signal is input to the terminal Rb to ground the piezoelectric element Pv through the switching circuit 99b to thereby rapidly drop the voltage applied to the piezoelectric element Pv. This produces a forward waveform Ha.
Also, after a HIGH signal is input to the terminal Rc to apply the voltage of the power supply 97 through the switching circuit 99a to the piezoelectric element Pv, then a HIGH signal is input to the terminal Rd to ground the piezoelectric element Pv through the constant-current circuit 98b. This produces a backward waveform Hb.
However, the first technique requires the waveform generator 95 and the power amplifier 96, and the second technique requires the constant-current circuits 98a and 98b and the switching circuits 99a and 99b, thus both facing the problems of circuit complexity and high cost.
Accordingly, drive devices with simpler circuit configurations are suggested (for example in Japanese Patent Application Laid-open No. JP2004-80964). The drive device of JP2004-80964 exercises drive control using three values (maximum, minimum, and mean values) of voltage applied to a piezoelectric element.
However, although allowing some degree of low-speed drive using the maximum, minimum, and mean voltage values, the drive device of JP2004-80964 still has difficulty in addressing lower-speed drive required in servo control or the like. In addition, it is also difficult to smoothly change the drive speed with a change in the direction of movement from the forward (normal) to the backward (opposite) direction.